1. Field of the Invention:
This invention relates in general to mechanisms for supporting a casing hanger in a wellhead and in particular to means for increasing the strength of the support. 2. Description of the Prior Art:
In subsea wells, a wellhead is located on the sea floor. A first string of casing will be lowered through the wellhead and supported on the wellhead by means of a casing hanger located at the top of the casing string. Additional smaller diameter strings of casing may be subsequently installed on the first casing hanger. Various mechanisms have been used in the past for supporting the casing hangers in the wellhead. These include single load shoulders, expanding latch mechanisms, and high friction devices.
The simplest and most economical method is to use a single load shoulder. The load shoulder is a conical shoulder located in the bore of the wellhead housing. The diameter of the bore will be smaller below the load shoulder than above. The casing hanger has a downward facing load shoulder which lands on the load shoulder in the bore.
A single load shoulder, however, has limited load capacity. The inner diameters of the wellhead housing bore both above and below the load shoulder have fixed requirements that limit the radial or transverse dimensions of the load shoulder. Also, the yield strength of the wellhead housing is typically only around 80,000 psi. This type of load shoulder is capable of withstanding the stresses imposed upon it due to loadings on the wellheads up to 10,000 psi working pressures. The problem is to have a shoulder capable of withstanding stresses imposed from working pressures up to 15,000 psi.